Fibers made of thermoplastic polymers such as polyesters and polyamides are excellent in mechanical properties and dimensional stability, and therefore are widely used not only for clothing applications, but also for home interior, car interior and industrial applications and the like, having very high industrial values. However, at present when applications of fibers are diversified, the properties required of fibers are diverse, and the existing polymers may not be able to respond to those required properties in some cases. If novel polymers that can respond to those applications are designed at the level of molecules, the problems of cost and time are a problem. Consequently the development of composite fibers having the properties of multiple polymers may be selected as the case may be. In these composite fibers, for example, a main component is covered with another component, to provide sensitive effects such as hand and bulkiness or mechanical properties such as strength, initial modulus and abrasion resistance which cannot be achieved by fibers of a single component only. Composite fibers come in a variety of forms and modes, and various techniques have been proposed for adaptation to respective applications of fibers. Among those composite fibers, active R&D is conducted on so-called “sea-island” composite fibers in each of which numerous island component fibers are disposed in a sea component.
A typical application of sea-island composite fibers is the production of ultrafine fibers. In this case, a slightly soluble island component is disposed in a soluble sea component, and from the obtained fiber or textile product with this configuration, the soluble component is removed to leave island component fibers as ultrafine fibers. In this case, extremely ultrafine fibers of the nano-order that cannot be produced by any single spinning technique can also be obtained. Ultrafine fibers with a single fiber fineness of hundreds of nanometers can be developed, for example, as artificial leathers and textiles exhibiting new feelings and senses by using the soft touch and delicateness unavailable from general fibers. In addition, the compact inter-fiber gaps are used to provide high-density woven fabrics usable as sports clothing requiring wind-breaking capability and water-repelling capability. The ultrafine fibers go into fine grooves and provide large specific surface areas, and the very fine inter-fiber voids can catch dirt. Therefore, ultrafine fibers exhibit high adsorbability and dust collectability. These properties are used for industrial material applications as wiping cloths and precision polishing cloths for precision apparatuses, etc.
The sea-island composite fibers as a starting material of ultrafine fibers include two major types. One is the polymer alloy type in which polymers are melt-kneaded together, and the other is the composite spinning type using a composite spinneret. Among these composite fibers, the composite spinning type is considered to be an excellent technique since the composite cross section can be precisely controlled by using a spinneret.
Techniques concerning the sea-island composite fibers of the composite spinning type include, for example, the techniques characterized by composite spinnerets disclosed in JP 8-158144 A and JP 2007-39858 A.
In JP '858, a soluble component polymer reservoir extended in the cross sectional direction is installed below the holes of a slightly soluble component, and the slightly soluble component is inserted into the soluble component polymer reservoir to produce sheath-core composite streams, the sheath-core composite streams then being joined and subsequently compressed, to be discharged from the final hole. In that technique, for both the slightly soluble component and the soluble component, the passage widths established between a diversion passage and introduction holes are used to control the pressures, to make the inserting pressures uniform, thereby controlling the amounts of the polymers discharged from the introduction holes. Making the pressures uniform of the respective introduction holes like this is excellent in view of controlling polymer streams. However, to keep the size of the final island component fibers on the nano-order, at least the polymer amount of each introduction hole at least on the sea component side is as very small as 10−2 to 10−3 g/min/hole, and therefore the pressure loss proportional to the polymer flow rate and the wall interval becomes almost 0. This makes it very difficult to control the polymers as the sea component and the island component precisely. In fact, the ultrafine fibers obtained from the sea-island composite fibers obtained in examples was approx. 0.07 to approx. 0.08 d (approx. 2700 nm), and ultrafine fibers of the nano-order were not obtained.
JP '858 indicates that if the compression and joining of composite streams in which a soluble component and a slightly soluble component are arranged relatively at equal intervals are combined multiple times, a sea-island composite fiber in which fine fibers of the slightly soluble component are disposed in the cross section of the composite fiber can be obtained. In that technique, certainly in the cross section of the sea-island composite fiber, the island component fibers may be regularly arranged in the inner layer portion. However, when each of composite streams is reduced in size, the outer layer portion is affected by shearing by the hole wall of the spinneret. Consequently, in the cross sectional direction of the reduced composite stream, a flow velocity distribution is generated, and the slightly soluble component fibers in the outer layer of the composite stream and those in the inner layer become greatly different from each other in fiber diameters and forms. In the technique of JP '858, to achieve island component fibers of the nano-order, the above-mentioned operation must be repeated multiple times before the final discharge. Therefore, the difference in the distributions of cross sectional forms in the cross sectional direction of the composite fiber may become very large as the case may be, and variations in island component fiber diameters and cross sectional forms occur.
In JP 2007-100243 A, as the spinneret technique, a known conventional sea-island composite spinneret using pipes is used, and the melt viscosity ratio between a soluble component and a slightly soluble component is specified so that a sea-island composite fiber with a relatively controlled cross sectional form can be obtained. Further, JP '243 indicates that if the soluble component is dissolved in a later step, ultrafine fibers with a uniform fiber diameter can be obtained. However, in that technique, the slightly soluble component divided into fine lines by pipes is once formed into sheath-core composite streams using sheath-core conjugating holes, and the composite streams are joined and subsequently reduced in size to obtain a sea-island composite fiber. The formed sheath-core composite streams are completely round in cross sectional form due to the surface tension acting after discharge from the conjugating holes. Consequently, it is very difficult to positively control the form. Therefore, there is a limit in controlling the cross sectional forms of the island component fibers, and complete circles and ellipses similar to complete circles exist together. With regard to this matter, even if the form of the hollow portion of each pipe is changed, the effect of this modification is small because of the influence of the surface tension of polymer streams. In the technique of JP '243, with regard to the variation of the circumscribed circles of the island component fibers, the circles can be made relatively uniform. However, it is very difficult to achieve a non-circularity and to make uniform the noncircular cross sectional form. Therefore, JP '243 is very limited for allowing the design of ultrafine fibers adaptable to applications and allowing the design of textile products composed of the ultrafine fibers.
In the case where the island component fibers have a completely circular or similar cross sectional form, if the fibers are simply woven and treated to remove the sea component, the ultrafine fibers with a circular cross sectional form contact each other at the tangential lines, and among the ultrafine fibers, gaps depending on the fiber diameter are formed. Further, the flexibility increases simply in response to the fiber diameter. Consequently, in the case of sports clothing, water permeates through the gaps to limit the waterproof performance. Furthermore, since the cloth is soft, such problems as displeasing stickiness and the increase of cloth weight occur as the case may be. Moreover, also in applications as wiping cloths and polishing cloths, since the ultrafine fibers have a completely circular or similarly elliptic cross sectional form, the dirt and abrasive may slip on the surfaces of the fibers. Moreover, ultrafine fibers raised on the surface layers by buffing or the like are soft and weak and therefore are limited in wiping performance and polishing performance, and in the case where the dirt and abrasives caught under ultrafine fibers are pressed at lines (tangential lines of circles), the material to be polished may be flawed unnecessarily as the case may be.
WO 89/02938 proposes a distribution type spinneret in which fine grooves and holes are used to form polymer passages, and conjugation is performed immediately before and/or immediately after discharge to form a complicated cross sectional form. In the spinneret of this type, depending on the arrangement of holes in the final distribution plate, two or more types of polymer streams can be arranged at arbitrary points in the cross section of the fiber. Further, by joining island component fibers together, island component fibers with a noncircular cross sectional form of the micron order or a diverse composite cross section composed of the joined fibers may be able to be formed.
However, in the case where island component fibers or ultrafine fibers of the nano-order are produced, it is necessary to divide one component polymer extremely, and in the distribution holes immediately before the discharge plate, the discharge rate per hole is as extremely small as 10−4 to 10−5 g/min compared with the micron order (10−0 to 10−2 g/min). Consequently, the pressure loss necessary for metering the amount of polymer is almost 0 kg/cm2, and the polymer metering capability is very low. From this point of view, in reference to the technique of JP '243, a filter or the like is used to apply a pressure loss so that the polymer passes through quite different passages after having been metered, and is divided till immediately above the discharge plate or till the discharge surface. Therefore, the discharge rates of the island component and the sea component become uneven from place to place, and it is very difficult to form a highly precise sea-island composite cross section. In particular, to produce ultrafine fibers (island component fibers) as described before, the discharge rate per distribution hole is very small. For this reason, in the technique of WO '938, it is difficult to obtain uniform ultrafine fibers in view of the precision of the sea-island composite cross section.
Further, in the passages (hole arrangement and grooves) presented as examples in WO '938 and in the description, the abnormal retention that some polymer streams become hard to flow is not taken into consideration. Therefore, in the case where a branch hole is closed halfway in a passage, the polymer does not flow through the branch hole on the downstream side at all, or the amount of the subsequent polymer stream is greatly decreased. Accordingly, in the technique of WO '938, if a branch hole is closed, all the polymer that should flow through the branch hole flows through other branch holes, and the cross sectional mode of the composite polymer streams becomes greatly different from the intended cross sectional mode. Further, when the composite polymer streams obtained by discharging from respective distribution holes and joining the discharged streams are compressed and discharged, it is not considered to protect the composite polymer streams. For this reason, the decline in the precision of composite cross section is further promoted.
It could therefore be helpful to provide a sea-island composite fiber that can be converted into ultrafine fibers having an extreme fineness of the nano-order, which, as island component fibers, have a non-circularity and are uniform in the noncircular cross sectional form.